Conventional equipment using linear vibration motors include vibration generators for informing of incoming calls by mechanical vibration, handy phones including such vibration generators, compressors for compressing and circulating gases or fluids, and reciprocating electric razors. The compressors and electric razors use the linear vibration motors as their driving sources.
A typical linear vibration motor has a structure of a single-phase sync motor. That is, the typical linear vibration motor it has a mover comprising a permanent magnet and a stator which is obtained by winding a coil around an iron core, and the mover reciprocates when an AC voltage is applied to the coil.
As described above, when generating vibration by a reciprocating motion of the mover, a strong electromagnetic force is needed. However, the energy that is required for driving the linear vibration motor can be minimized by forming a spring vibration system which includes the mover and a spring member supporting the mover. That is, in the linear vibration motor in which the mover is supported by the spring member, the spring vibration system including the mover is vibrated at its natural resonance frequency, whereby the linear vibration motor can be driven with a relatively low energy.
As a method for controlling an output of the linear vibration motor in which the mover is supported by the spring member, the amplitude value of a voltage or current which is supplied to the linear vibration motor may be adjusted while driving the linear vibration motor with its resonance frequency (for example, refer to Japanese Published Patent Application No. 2001-193993).
In the linear vibration motor, however, when the stroke length of the mover becomes larger than a predetermined allowable value, a problem such as a collision between the mover and the motor body or a breakage of the support spring may occur. Therefore, the stroke length of the mover is restricted by the structure of the linear vibration motor.
For example, when the extension of the support spring exceeds a predetermined value due to an increase in the stroke length of the mover, the support spring may plastically deform and break. Further, when the stroke length of the mover is increased to about the size of the motor body in the mover vibrating direction, the mover may collide against the inner wall of the motor body and break.
Accordingly an apparatus has been proposed for driving a linear vibration motor which solves the above-mentioned problems. This linear vibration motor driving apparatus is provided with a detector such as a position sensor for detecting the position of the mover of the linear vibration motor, and reduces the output of the linear vibration motor when the stroke length of the mover exceeds a predetermined allowable value, that is, decreases the amplitude value of the voltage or current applied to the linear vibration motor, thereby preventing the linear vibration motor from being destroyed due to a collision between the mover and the motor body or extension of the support spring over a critical value (for example, refer to Japanese Published Patent Application No. Hei.11-324911).
However, since the conventional linear vibration motor driving apparatus (hereinafter also referred to as a motor driving apparatus) drives the linear vibration motor with the frequency of the reciprocating motion being maintained at the resonance frequency of the spring vibration system including the mover, adjustment of the output of the linear vibration motor is carried out by only the stroke length of the mover. As a result, the maximum output of the linear vibration motor is undesirably restricted by the structure of the linear vibration motor, and further, the maximum output of the linear vibration motor is undesirably restricted by the power supply voltage which is applied to the motor driving apparatus.
The restriction on the motor output by the structure of the linear vibration motor will first be described in detail.
In the linear vibration motor, the maximum stroke length of the mover can be increased up to only the shorter one between the length of the body of the linear vibration motor containing the mover in the mover vibration direction, and the length corresponding to the elastic limit of the mover supporting spring.
Accordingly, in order to increase the maximum output of the linear vibration motor, the size of the motor body in the mover vibrating direction should be increased so as to secure a larger stroke length of the mover and, further, a spring having a larger elastic limit length should be used as the mover supporting spring. Alternatively, the spring constant of the mover supporting spring should be increased so as to increase the resonance frequency of the linear vibration motor.
Accordingly, in the conventional linear vibration motor, the mechanical construction is determined based the required maximum output, and therefore, an increase in the maximum output may lead to not only an increase in size but also a reduction in motor efficiency in an output region of the highest frequency of use, i.e., a reduction in the ratio of the motor output to the motor input.
The above-mentioned problem will be specifically described taking, as an example, a case where the linear vibration motor is applied to a compressor of an air conditioner. In this case, an output region of the highest frequency of use is not a high output region where a high motor output is generated for rapid heating operation or rapid cooling operation but a low output region where the motor output is about 10-20% of the motor output in the high output region. In the low output region, since the stroke length of the mover is reduced, the motor efficiency is reduced. Further, in the compressor, the top clearance is extended due to a reduction in the stroke length of a piston, leading to a reduction in work efficiency.
The restriction on the motor output by the power supply voltage of the linear vibration motor will now be described in detail.
In the conventional motor driving apparatus described above, the value of the applied voltage is adjusted by intermittently applying the driving voltage to the linear vibration motor so that the mover has a desired stroke length. To be specific, when the output power that is required of the linear vibration motor is increased, the value of the voltage which is applied to the linear vibration motor is increased so as to increase the stroke length of the mover.
However, when a general inverter is used for the motor driving apparatus, the motor driving apparatus cannot output an AC voltage whose amplitude value is larger than the voltage level of an inputted DC voltage. In other words, even when the amplitude value of the driving voltage which is applied to the linear vibration motor is increased so as to increase the stroke length of the mover, the motor driving apparatus can apply, to the linear vibration motor, only an AC voltage whose amplitude value is equal to or lower than the voltage level of the input voltage. As a result, the maximum output of the linear vibration motor is restricted by the voltage level of the DC voltage which is applied to the motor driving apparatus.
In this case, in order to increase the maximum output of the linear vibration motor, there is no choice but to reduce the number of windings of the coil which is a component of the stator of the linear vibration motor. That is, by reducing the number of windings of the coil, the magnitude of an induced voltage that is generated by the linear vibration motor is changed, whereby the balance between the driving voltage and the driving current, i.e., the driving power that is a product of the driving current and the driving voltage, is changed.
Accordingly, in the conventional linear vibration motor, the number of windings of the coil as a component of the stator of the linear vibration motor is determined based on the required maximum output, thereby leading to a possibility that the motor efficiency might be reduced in the output region of the highest frequency of use.
For example, when the number of windings of the motor coil is reduced to increase the maximum output of the linear driving motor, the amount of current in the output region of the highest frequency of use, i.e., in the output region where the motor output power is low, is increased, thereby resulting in a reduction in motor efficiency due to an increase in copper loss or core loss in the motor, or an increase in loss in the inverter.
The present invention is made to solve the above-described problems. Accordingly, an object of the present invention is to provide a motor driving apparatus which can control the motor output in the state where the voltage level of the driving voltage of the linear vibration motor is kept constant, thereby to facilitate output control for the linear vibration motor, and which can increase the maximum output of the linear vibration motor without modifying the specifications of the linear vibration motor or the power supply thereof.